The overall objective of the proposed research is to define the mechanism of human T-cell leukemia virus (HTLV) replication and transformation. HTLV is associated with specific human T-cell leukemia/lymphomas, endemic to certain regions of the world. HTLV has two major biological features which distinguish it from most other animal retroviruses: (a) a limited tissue-range for its replication due at least in part to cis-acting functions of the HTLV long terminal repeat (LTR); (b) acute transformation of human T-lymphocytes in vitro without having sequences related to normal human DNA sequences (viral oncogenes). We have established the necessary basis to begin a molecular genetic study of these properties of HTLV. Molecular clones of the provirus of the type II human T-cell leukemia virus (HTLV-II) have been isolated and DNA transfection assays for HTLV DNA have been developed. We will use in vitro mutagenesis and recombinant DNA methods to map regions of the viral genome responsible for these unique properties of HTLV. The results of these experiments will provide the conceptual basis and the molecular tools for future functional studies. The specific aims are: 1. To determine cis-acting functions of the HTLV long terminal repeat (LTR) which confer tissue specificity to HTLV replication. We will investigate whether novel sequences present in the LTR act as tissue-specific "enhancers" of transcription, by formation of in vitro recombinants of these LTR sequences with heterologous promoters and by in vitro deletion mutagenesis. 2. To determine whether trans-acting functions of HTLV are required for transcriptional function of the HTLV LTR. If so, then we will determine whether a region of unknown function, termed the pX region, is involved by in vitro deletion mutagenesis. 3. To characterize the regions of the HTLV genome necessary for in vitro transformation of human T-lymphocytes. We will obtain mutants of HTLV deficient in transformation by in vitro recombinant DNA methods and by isolation of spontaneously arising HTLV mutants. We will also attempt to construct a virus which carries only the pX region and determine if the pX region is sufficient for transformation.